1. Field of the Invention
The present invention relates to an image reader, an image forming apparatus, and a method of correcting image data.
2. Description of the Related Art
In the related art, as a reader of a copying machine or a facsimile, a scanner for computer input, or the like, an image reader is used which automatically reads image information of a document. In this type of image reader, light is irradiated onto the document using a light source installed to extend in a main scanning direction perpendicular to the conveying path of the document, and reflected light from the irradiated document is received by an image sensor. In this manner, an image on the document is read. In recent years, for the purpose of reducing the size of the apparatus, the following image reader comes into practical use. That is, in the image reader, a light-emitting diode (LED) having a small shape is used as a light source, and included therein is a contact image sensor (CIS) type scanning unit configured to form an image on a linear image sensor through an optical system of an unmagnification imaging system.
Image data read by the image reader includes irregularity of the light source and sensor sensitivity irregularity. For this reason, shading correction is executed to eliminate the influence of the irregularity of the light source or the irregularity of the sensor sensitivity using data (shading data) obtained by scanning a predetermined reference surface by the scanning unit. Shading correction is generally carried out by the calculation of the following expression.Dout=(Din−Bk)/(Dsh−Bk)×(2n−1)    Dout: output image data after shading correction    Din: image data obtained when the document is scanned    Dsh: image data (shading data) obtained when the reference surface is scanned    Bk: black level (image data level when light is not input)
An image reader is already known which includes a white member arranged to face the scanning surface of the scanning unit and scans the surface of the white member by the scanning unit during a period other than a document scanning period with the surface of the white member as a reference surface, thereby generating shading data (for example, see Japanese Patent Application Laid-open No. 2005-328156 or the like).
On the other hand, in the optical system of the unmagnification imaging system, the optical path is short and the degree of condensing of light from the light source is high. For this reason, the amount of change in the output level increases with change in the distance between the scanning unit and the document surface (hereinafter, the amount of change in the output level with the change in the distance between the scanning unit and the document surface is defined as an illumination depth characteristic). Meanwhile, since the reference surface which is used in generating shading data is generally fixed, the distance between the scanning unit and the reference surface is constant. Thus, if the output level changes at a certain position in the main scanning direction due to the influence of the illumination depth characteristic caused by flapping of the document when conveying the document, it is not possible to eliminate the change amount through shading correction. The change in the output level that is continuous in the sub scanning direction causes a problem in that a vertical streak occurs in an image.
In the image reader described in Japanese Patent Application Laid-open No. 2005-328156, in generating shading data, the white member is moved to a position facing the scanning surface of the scanning unit to retract the white member during the document scanning period. For this reason, it is considered that the size of a space (hereinafter, referred to as a gap) between the scanning surface of the scanning unit and the white member when the white member is moved to the position facing the scanning surface of the scanning unit is set to be smaller than the size of the gap (that is, the range in which the document is flapped) when the document is scanned, such that it is possible to reduce the influence of the illumination depth characteristic through shading correction.
The image reader described in Japanese Patent Application Laid-open No. 2005-328156, for example, in reading continuous documents, it is necessary to generate shading data by repeatedly moving the white member to the position facing the scanning surface of the scanning unit. There is a novel problem in that it takes a lot of time to move the white member, resulting in degradation in productivity (the number of sheets to be read for a predetermined time).